1. Field of the Invention
The present invention relates to a flyback converter for a power supply device, such as a switching mode power supply (SMPS), and more particularly to a variable-frequency flyback converter with a synchronous rectification function which is capable of detecting a secondary voltage of a transformer to determine whether the current load state is a mini load state with a predetermined load or less and preventing backward current from being generated in the mini load state, so as to reduce power consumption in the minim load state, including a no-load state.
2. Description of the Related Art
As well known to those skilled in the art, power supplies are modular power supply devices that convert an external supply voltage into voltages appropriate to various electric appliances such as a computer, a color television (TV), a video cassette recorder (VCR), an exchange and a wireless communication terminal, and act to cut off high frequencies above a commercial frequency using semiconductor switching characteristics to remove damage resulting therefrom.
Such power supplies can be roughly classified into linear power supplies and SMPSs. The SMPSs are more advantageous for miniaturization and higher in efficiency than the linear power supplies so that they can be more appropriate to lightness/smallness of associated electronic products. In this regard, the SMPSs have taken their place as main power supply devices of electronic products since the beginning of the 1990's.
The SMPSs can be variously classified according to the types of circuits and the types of input and output voltage sources. The newest technical one of the circuit types is a resonant type. The most often used ones of the voltage sources are an alternating current/direct current (AC/DC) converter that converts an AC supply voltage of 110V or 220V into a DC voltage of 5 to 48V, and a DC/DC converter that reconverts that DC voltage into a DC voltage of 3.3 to 48V.
These SMPSs are advantageous for lightness and smallness, but have the problem of having to attenuate switching noise and electromagnetic waves occurring in a switching transistor, etc.
Meanwhile, as one of flyback converters for the SMPSs, a variable-frequency flyback converter is used in which a switching frequency for zero-crossing switching varies with a load state. This variable-frequency flyback converter is adapted to detect zero-crossings of secondary current of a flyback transformer and control switching timing of a main switch connected to a primary winding of the transformer in accordance with a result of the detection. The switching frequency of this flyback converter varies with the load state.
That is, if the load is large, the secondary current is large, too. As a result, the zero-crossing time is lengthened, thereby causing the switching frequency to be lowered. On the contrary, if the load is small, the secondary current is small, too. As a result, the zero-crossing time is shortened, so the switching frequency is raised.
However, in a mini load state where the load is minimal, the switching frequency is raised, causing an increase in switching noise. For this reason, in the mini load state, the upper-limit value of the switching frequency is set to a predetermined value, for example, 150 kHz, in order to suppress the increase of the switching noise to a certain degree.
Here, a distinction may be made between the normal load and the mini load on the premise that a load in a standby mode is defined as the mini load. In this case, a load of 3 W or less, including no load, can be defined as the mini load.
Notably, in the mini load state, a discontinuous conducting mode (DCM) period in which no primary current and secondary current flow exists because the upper-limit switching frequency value is set. In the DCM period, a valley is generated due to a resonance by parasitic capacitors of the primary winding and main switch after a voltage across the main switch is transferred to a secondary winding of the flyback transformer. This valley includes a high voltage causing backward current, so there is a need for a scheme to prevent the backward current.
Such a conventional flyback converter for an SMPS is shown in FIG. 1.
FIG. 1 is a circuit diagram of a conventional flyback converter.
In FIG. 1, if the gate of a thyristor M1 is turned on, an input voltage is stored in a primary winding of a transformer T. Thereafter, if the gate of the thyristor M1 is turned off, a positive voltage is applied to a negative electrode of an auxiliary secondary winding N1 of the transformer T. This positive voltage is in turn applied to a MOS (Metal-Oxide Semiconductor) transistor SR through a diode D1 and resistor R1 to turn it on. As a result, a positive voltage is applied to a negative electrode of a main secondary winding NS of the transformer T to provide an output voltage Vout.
At this time, a current transformer TA detects backward current and, when the backward current is detected, a positive voltage is applied to a negative electrode of a secondary winding of the current transformer TA. This positive voltage is in turn applied to a controllable unit S1 through a resistor R2 to turn it on. As a result, the MOS transistor SR is turned off to block the backward current.
However, the above-mentioned conventional flyback converter can somewhat reduce power consumption because the MOS transistor SR is turned off when the gate of the thyristor M1 is turned on, but has a disadvantage in that the MOS transistor SR still has considerable power consumption in a no-load state.